Michelle Beckett, a 44-year-old entrepreneur from Harrogate, always knew she was different. But, like many women who suffer from attention deficit hyperactivity disorder (ADHD), she did not fit the stereotypical profile of a child with the condition. Beckett’s difficulties would come later in life, when she failed to live up to her academic potential, experienced two failed marriages and had mental health problems.

At 36, Beckett decided to seek help. A neuroscientist in York told her that the results of an EEG – a recording of brain activity – suggested she had ADHD. “I dismissed his diagnosis as rubbish,” she says. “How could I have ADHD? I was just [being] crap and needed to sort myself out.”

Beckett’s problems escalated from there. She experienced mood swings so dramatic that any criticism could cause her to go from feeling happy to contemplating suicide. After a failed business venture, she ended up in the care of a local mental health crisis team, believing that her “family and the world would be far better off without me”.

It was then that ADHD was mentioned again by a psychiatrist, and, at the age of 44, she decided to seek a private diagnosis; one that not only transformed her life but saved it: “Everything fell into place. I wasn’t crap because I found VAT returns painful, blurted out stuff and was messy. I wasn’t crap at all. I have a neuro difference, which gives me many advantages.” Beckett is just one of many people who slipped through the diagnostic net as children and were left to cope with their ADHD into adulthood.

It’s now 30 years since the American Psychiatric Association agreed that ADHD was the best term to describe this condition. It has long been viewed as something that predominantly affects children – mostly young boys – but increasing numbers of adults are now being diagnosed. According to the Royal College of Psychiatrists, ADHD is indeed more common in boys than in girls – surveys of children between the ages of five and 15 years have found that 3.62% of boys and 0.85% of girls had ADHD. As many people get older, however, the condition improves and around 1% of adults continue to have the condition, with about 2-4% showing partial symptoms.

ADHD in adults has come under the spotlight this year with new advances in clinical research. A BBC Horizon programme featured comedian Rory Bremner’s personal journey to understand the science behind the condition. And the thinktank Demos has started a research project exploring the social and financial impact of undiagnosed ADHD in adults. It is due to be completed this year.

I was diagnosed with ADHD when I was six. I doubt my psychiatrist felt much ambivalence when he was diagnosing me. During one of the assessments, instead of focusing on arranging a series of pictures into chronological order, I thought it would be more productive to climb to the top of the bookshelves in his office. I was easy to diagnose, but many children with ADHD aren’t bouncing off the walls or scaling bookshelves and are therefore much less likely to receive a diagnosis in childhood. This is especially true for women, whose ADHD tends to manifest in less overt ways than it does in boys.

Dr Ashok Roy, head of the Royal College of Psychiatrists’ intellectual disability faculty, says the core symptoms are the same in men and women, but that “female patients’ inattention problems often are combined with daydreaming, whereas men have more hyperactivity and behavioural problems, which are more noticeable during childhood”. Roy has also observed that women with ADHD also tend to suffer from other disorders that can affect their mood and behaviour.

Arielle, a 27-year-old writer from Boston, was diagnosed with ADHD when she was 19. She remembers always being a quiet child: “I didn’t show the classic signs people think of when they hear about hyperactivity. Instead of bouncing off the walls, I simply fidget constantly. I slept through most of school, but, if I was awake, I never was an attentive listener,” she says.

Over the years, I started to assume I was just lazy

Arielle says her grades suffered throughout school because she was unable to focus on her homework. Her teachers told her that if she just did her homework, stayed awake in class and became organised, she could be a straight-A student. “Over the years, I started to assume I was just lazy. Instead of thinking there might be something wrong biologically, I assumed it was all my fault. And, every time I’d resolve to be better at being attentive in class, or neat or diligent about homework, I’d inevitably fail.”

Arielle feels that going undiagnosed until adulthood was “pretty harmful” for her, and sometimes tells herself that if she really had ADHD, someone would have noticed it sooner: “I still frequently doubt that I have ADHD and sometimes believe I’m just fooling everyone in an attempt to excuse my forgetfulness, messy nature, issues with planning ahead and problems completing projects.”

It’s not surprising that Arielle feels this way, as there is a prevailing scepticism about the condition, even among psychiatrists. The most frequent criticism levelled at the diagnosis of ADHD is that it pathologises normal behaviours. The neurologist Dr Richard Saul promoted this idea in his 2014 book, ADHD Does Not Exist. At the same time, in an article for Time magazine, Saul expressed concern over the stimulants used to treat ADHD – most commonly methylphenidate – and argued that, under the criteria employed to diagnose the condition, “the entire US population could potentially qualify. We have all had these moments and, in moderate amounts, they’re a normal part of the human condition.”

These arguments tend to come as a reaction to the recent explosion of ADHD diagnoses in children, particularly in the US. However, Roy argues that “untreated populations fare worse on academic, social and occupational outcomes as well as contact with the law”. He argues that practitioners should “guard against overdiagnosis by the use of systematic assessments using internationally agreed criteria”.

In April this year, the Lancet published a report that suggests the condition is a valid pathology. Researchers from nine countries scanned the brains of people aged between four and 63 years old, 1,713 with ADHD and 1,529 without. The report found small differences in brain size between those with the condition and those without it. These differences were more pronounced in children, and the differences were still apparent in unmedicated subjects.

If we accept that ADHD is real, then we might want to address the lack of awareness that exists within our society, as undiagnosed adults are often left to live dysfunctional lives. According to Roy, part of the problem when diagnosing adults is that they are more likely to “internalise their inner restlessness rather than act it out as children do”. Roy adds: “This may manifest as mood swings, difficulty getting along with others, problems holding down a job, substance-abuse problems, and a pattern of being irresponsible and making poor decisions. These behaviours may be incorrectly attributed to personality problems, anxiety or depression rather than ADHD.”

Victoria Jones … ‘It has been a revelation to understand it.’ Photograph: Jon Super for the Guardian

Indeed, the most common story I hear from those I speak to with ADHD is that they were diagnosed with conditions such as depression and anxiety before they discovered they had it. This is something I can attest to. I stopped receiving treatment for ADHD in my teens. This was partly because I moved around so much but also because I was resistant to the label; I didn’t want to believe there was something wrong with me. In my early 20s, I sought treatment again only because my life was falling apart. I didn’t receive the support I felt I needed and was given antidepressants and anti-anxiety medication instead.

Victoria Jones is a very amiable student from Lancashire who begins talking to me a mile a minute in a way that would defeat the deftest shorthand practitioner. Like many people with undiagnosed ADHD, she had been treated for anxiety and depression for 10 years prior to her diagnosis. Before being diagnosed, Jones led a chaotic life, experienced many failed relationships, flitted from one job to the other, and eventually ended up owing £25,000 through impulse spending and her inability to manage her debts.

Since being diagnosed with ADHD at age 34, she has been on “a rollercoaster” of emotions and learning: “Having been one of those people who thought it was an excuse for boys’ ‘naughty’ behaviour, it has been a revelation to learn about and understand it.” She is involved in an ADHD support group in Lancashire and wants to raise awareness about the condition: “I wish more than anything that the media, schools and other agencies were educated better on the topic and it became as understood and supported as autism. I think this would give families, friends and the rest of society the means to assist in and improve the lives of those who have it.”

John, 30, from the home counties, says he has struggled with insomnia since childhood and has experienced problems in his personal and professional life due to his forgetfulness: “My relationships always suffer – I have lost count of the number of friends from my teens and early 20s I no longer speak to due to being useless at keeping in touch, forgetting to respond to messages or simply not remembering that they exist.” John adds that this applies to everything in his life: “If it’s beyond my very narrow, immediate focus, it gets ignored, even consciously put to one side. I will step over piles of urgent laundry to complete my pointless DIY project. I will pay late fees on bills I can easily afford just because it seems that paying them will take me several hours, and I really want to do something else.”

I will step over piles of urgent laundry to complete my pointless DIY project

John was finally diagnosed quite recently and, like many adults with ADHD, he became aware of the condition through his own research. He was assessed and diagnosed relatively quickly, but has mixed feelings about the treatment he has received: “I’ve been on methylphenidate since being diagnosed a few months ago. Initially, I thought it was a gamechanger – I could sit down and complete something without getting distracted.” But more recently John has begun to have doubts: “I notice I can be a little too focused, energetic and positive, and if I am doing something pointless when my meds kick in, that can be a whole day wasted.”

Andrew Kavanagh, 44, an animator from Dublin, didn’t receive a diagnosis until he was 43. “I often wondered why I had so many challenges in achieving what others found relatively easy to do,” he says. He only suspected he might have the condition when he met the neuroscientist Áine Behan, who, as CEO of Cortechs, was developing gaming technology to help children with ADHD improve their focus. “I knew little about the condition at the time, but as my research into the area deepened, I recognised many of my own traits in the symptoms attributed to ADHD sufferers.”

Kavanagh describes himself as fidgety, often impatient, a poor sleeper and someone who struggles to complete tasks. “I often feel like when I start something, I should be able to do it immediately rather than having to go through a learning process. I’ve always been impatient for results and often start a lot of different things and quickly become overwhelmed.”

Roy says that many adults remain undiagnosed because psychiatrists do not receive enough training. He believes the government needs to invest more in this. Labour’s shadow minister for mental health, Barbara Keeley, agrees. “Adults with ADHD who are undiagnosed and untreated in childhood are at greater risk of low self-esteem, substance abuse, anxiety and depression,” she says. “Investing in children’s mental health services and in early intervention is key to preventing this spiral.” She adds that it is “deeply worrying” that Health Education England is cutting back on training courses for GPs and trainees.

The Department of Health describes Keeley’s comments as “completely misleading” and adds that since 2005 the government has made an additional £250m available each year for children’s mental health services, and that by the end of 2017 it intends to publish a green paper setting out plans to improve access and the provision of services.

It’s not clear if ADHD should be considered an illness. It really describes a different type of brain, and may be partially responsible for the achievements of many successful people, including Albert Einstein and Katherine Ellison. Despite the serious problems that adults with undiagnosed ADHD face, many living with the condition say it has had a positive impact on their lives too. As well as being inattentive, their ADHD allows them to hyper-focus, and their impulsivity can inspire creativity.

Beckett says she wouldn’t change her ADHD. “I can be quick-thinking, creative, adaptable, warm, enthusiastic, authentic, honest, spontaneous, ambitious, driven to change the world, feisty and outspoken. I am interested in many different types of people, many different issues and many concepts and ideas. I strongly believe I wouldn’t have these traits if I didn’t have ADHD.”

Kavanagh has a similar outlook and likens ADHD to a “clockwork toy wound up and set loose in a labyrinth without a map. You run in to a lot of walls, but occasionally you break through some of them.” He, too, says he wouldn’t change his condition: “Despite all the hard knocks I have encountered getting to where I am, I wouldn’t trade ADHD for anything. It’s part of what makes me who I am, and I embrace that.”

Some names have been changed. The charity Addiss offers help and advice for people who wish to find out more about ADHD.